1. Field of the Invention
The invention relates to a continuously operating press for the production of chipboards, fiberboards and plywood boards.
2. Description of the Related Art
In the case of presses operating in the high-pressure range, development is moving toward exerting higher surface pressures on the material being pressed. This is especially true if the purpose is to produce highly compressed chipboards, where the pressures range from 55 bar and above. With rolling support provided by means of rolling bars, these high pressures increasingly result in "Hertzian stresses" on the surfaces of the heated platens. On the other hand, the heated platens have to be produced from a weldable steel material, because it is necessary to weld in inserts appropriately at the deflection channels and generally seals at the outer end faces. Weldable steels have, however, only a limited carbon content, which normally results in a surface hardness of about 180 to 190 Brinell. Additional surface hardness treatments merely produce an increased Brinell hardness in the range from 200 to 220 Brinell. With the large dimensions of the heated platens (for example 2.5 m.times.10 to 15 m long), there is a thermal distortion which occurs during the course of the heat treatment for increasing hardness. Thus, there is a risk in such heat treatment processes that, if the heated platens are not rolled with a level surface area, the relatively thin hardened layer is removed in the subsequent grinding machining processes, thereby penetrating and exposing layers of lesser Brinell hardness.
In the practical operation of continuous presses using such heated platens, the above-stated condition has the effect that considerable wear occurs on the heated platens' supporting surfaces after operating hours of about 3000 to 6000 hours. This wear does not result so much in a general removal of the supporting surface, but instead, grooves (with peak and valley) are formed transversely to the through-running direction of the material to be pressed. These grooves correspond approximately to the spacing of the rolling bars. This results in increased running noises, and with increasing wear, produces critical vibrations in the overall press system. At a surface pressure of 50 bar, the "Hertzian stress" with use of rolling bars (in the diameter range around 20 mm) lies at about 200 Brinell. Thus, with the slightest disturbances, for example, in a regular and uniform lubrication distribution system, the system operates unreliably as a result of which the wear phenomena described above occur.
It is of disadvantageous significance in this case that the orthogonal running of the rolling bars in the pressing area is not ensured and that it is possible for the rolling bars to run into each other and even be destroyed.
From DE-P 23 43 427, a press of the aforementioned type is known, wherein several pressure bodies (abutments of press ram and platen) are located in succession in the direction of passage and between the pressure bodies and the rolling body chain, stationary, elastically bending roll off surfaces are provided. The rolling body chain consists of numerous small rollers, threaded onto rods extending over the width of the press. Between the individual rollers, located on rods off-set relative to each other in the longitudinal and transverse direction, straps are provided to connect with the subsequent rods. The elastically bending roll off surfaces consist of a plurality of strips of a precision flat rolled steel, in the longitudinal joints of which the noses of the straps of the rolling body chain are guided.
The purpose of the strip roll off surfaces is to bridge over the numerous roller bodies to insure the avoidance of pressure peaks which appear in the case of different press masses in the entering wedge and which could damage the steel belts and the roller bodies.
This known press and sizing apparatus cannot assure that the rollers will move in an optimum manner and without friction through the press gap without hindrance since the numerous strap noses grind against the longitudinal joints and the lateral surfaces of the roll off strips, thereby considerably reducing the advantage of the rolling support. In addition, the roll off strips would also be exposed to the aforedescribed wear after a few hours of operation and exhibit a certain corrugation harmful to the operation of the press.
Heating platen systems are known from German published patent Application DE 37 43 933, wherein heating zones are formed by means of longitudinal bore holes transversely to the direction of transport of the material to be pressed, whereby a difference in temperature profiles may be established. Thus, for example by raising the temperature in the center of the heated platen, better vaporization is obtained due to the higher vapor pressure from the inside to the outside, together with a purposely centered heat supply into the material to be pressed, in the production of narrower particle boards. In the course of the conversion from wide particle boards to smaller ones, energy consumption will be lower and the dish effect of the steel band resulting from differential thermal expansion may be equalized.
However, the heated platens simultaneously also constitute the rolling support of the rolling bars. In view of this, the transitions have corresponding toothed configurations. The transitions forming the three heated platens are laid out appropriately with thermal expansion gaps. This leads for the rolling bars to partially increased surface pressures in the transition range from one heated platen to the other, whereby in particular in the high pressure range the permissible specific surface pressure for the compression of the particle cake is further minimized, so that within this area partial transmission pressures of merely about 30 bars are permissible. Technologically, however, pressures of up to 50 bars are required. Due to the fan-shaped zigzag transitions in the transition range, very high production costs are encountered in the preparation for the installation of connections of the longitudinally drilled heated platens. The purpose and function of the longitudinally drilled heated platens according to DE 37 43 933 is to be able to produce narrower particle boards by means of the directed and centered supply of heat. This would also enable the exploitation of the advantage of using higher compression pressures for boards with higher (scatter) densities in view of the narrower width. However, this cannot be realized by reason of the limited permissible specific pressures. In actual operation with continuous presses, the use of such heated platens results in that, after a number operating units totalling about 3000 to 6000 hours, a marked wear of the support surfaces is encountered. This wear leads not so much to a general erosion of the supporting surface but more to the formation, in the direction of the passage, of transverse grooves generally corresponding to the spacing of the rolling bars (with valleys and peaks). In combination with this, increasingly loud running noises appear, which with progressive wear lead to critical variations of the overall press system.